The present invention relates to the general field of fuel flow in a turbine engine. The invention applies to any type of aviation turbine engine, and more particularly to airplane turbojets.
In an aeroengine, it is common practice for fuel to be used not only for burning in the combustion chamber of the engine, but also as a hydraulic fluid in hydraulic actuators for controlling pieces of variable-geometry equipment of the engine (such as air bleak valves, and valves enabling the geometry of the compressor of the turbine engine to be adapted).
For this purpose, the fuel circuit of a turbine engine typically comprises a main fuel line for feeding the combustion chamber of the engine with fuel, and an auxiliary fuel line connected to the main fuel line and serving to feed fuel to the hydraulic actuators in order to control pieces of variable-geometry equipment of the engine.
The auxiliary fuel line for feeding the hydraulic actuators includes electrohydraulic servo-valves, i.e. hydraulic valves that are controlled by servo-motors. These servo-valves serve to deliver a calibrated flow rate of fuel to one or the other of the chambers of a hydraulic actuator, and they are controlled by the electronic control unit (ECU) of the turbine engine.
It is known that servo-valves operate correctly providing the fuel is above a certain temperature. Unfortunately, in extremely cold weather, the temperature of the fuel fed to such servo-valves might be negative when the airplane is starting or taking off, and that runs the risk of the ducts of the servo-valves becoming clogged.
One of the certification requirements to which engine manufacturers are subjected thus requires systems to operate properly under extremely cold conditions (e.g. “icing” conditions). This implies that, in order to protect sensitive elements such as servo-valves, the temperature inside the ducts of the servo-valves must be greater than the point at which water dissolved in the fuel freezes.
In order to comply with such certification, it is known to place an oil or air heat exchanger upstream from the servo-valves in the auxiliary fuel circuit in order to heat the fuel entering the servo-valves and thus avoid any risk of their ducts clogging with ice formed by the water contained in the ice-cold fuel. With an oil heat exchanger, heat from the oil circuit of the engine is used for heating the fuel entering the servo-valves by heat transfer. With an air heat exchanger, the heat exchanger is located upstream from the low pressure pump and it is air taken from the compressor or the fan of the engine that is used for heating all of the fuel.
The use of an oil or air heat exchanger for heating fuel on entry into servo-valves nevertheless leads to numerous drawbacks. In particular, an oil heat exchanger is relatively heavy, there is a risk of fuel leaking into the oil, and there are hydraulic constraints in the oil circuit. With an air heat exchanger, drawbacks such as heavy weight, large size, and difficulty of installation and control are likewise present.